Open bottom fiber reinforced precast concrete arch unit

ABSTRACT

A one-piece open bottom fiber reinforced precast concrete arch unit is buried in compacted soil and has an arcuate top wall portion integrally connecting arcuate side wall portions. The fiber reinforced wall portions have a configuration which provides ductility and flexure strength for effectively utilizing the surrounding compacted soil to resist deflection of the unit in response to a load and to redistribute stresses in the unit. Each of the side wall portions of the unit has a reducing wall thickness towards its bottom and has a radius of curvature greater than the rise of the unit and at least twice the radius of curvature of the top wall portion. Bands of continuous fibers may be attached by adhesive resin to the outer surface of the arcuate side and top wall portions of the unit near opposite ends of the unit to increase the load carrying capacity of the unit.

BACKGROUND OF THE INVENTION

This invention relates to open bottom precast concrete arch units of thetype constructed to be buried in the ground or soil and as generallydisclosed, for example in U.S. Pat. No. 3,482,406, No. 4,558,969, No.5,281,053, No. 6,161,342, No. 6,205,717, No. 6,406,220, No. 6,408,581and No. 6,640,505. Such precast concrete arch units are produced invarious sizes and spans, for example, by Bebotech Corporation inMiddletown, Ohio who offers part elliptical and part circular shapes andas a single or one-piece casting or as a twin or two-piece casting withspans from 12 feet to 84 feet. In such precast concrete arch units, itis common to have a wall thickness of 8 to 14 inches and to haveembedded in the concrete walls steel reinforcing bars or rods which maybe in the form of a mesh. The steel reinforcing bars or rods provide theconcrete arch unit with sufficient strength to support the soil abovethe arch unit and any load which may be applied to the arch unit, forexample, by the wheels of a vehicle on a road passing over the archunit.

It is desirable to minimize the weight of a precast concrete arch unitwithout sacrificing strength for supporting a load in order to reducethe construction cost of the unit and the costs for handling,transporting and installing the unit at a construction site. Thus it isdesirable to reduce the volume of concrete and the volume of reinforcingsteel within a precast concrete arch unit in addition to reducing thelabor required to precast the unit. Furthermore, when a one-piece openbottom precast concrete arch unit is used, it is desirable for the unitto be transported while resting on one end and be nested with othersimilar arch units for efficiently transporting a plurality of the archunits on a semi-trailer bed. While an arch unit may be precasted in twoor more sections to facilitate transportation, substantial additionaltime and labor is required to assemble and connect the sections at thesite where the arch units are installed.

SUMMARY OF THE INVENTION

The present invention is directed to an improved one-piece open bottomprecast concrete arch unit which is designed for underground use and tobe buried in compacted soil. The arch unit of the invention provides allof the desirable features mentioned above and is effective to utilizethe surrounding soil mass as a resistance support for the arch unit andthereby minimize the production cost of the arch unit by substantiallyreducing the volume and weight of concrete and the weight ofreinforcement within the arch unit. The arch unit of the inventioneliminates the use of steel reinforcing bars or rods and thus eliminatesthe labor required to position steel reinforcement bars or rods betweenthe forms which are used to precast the arch unit.

In accordance with one embodiment of the invention, a one-piece archunit is precast with opposite arcuate side wall portions integrallyconnected by an arcuate top wall portion. The side wall portions have aradius of curvature substantially greater than the radius of curvatureof the top wall section, and the radius of curvature of the top wallportion is substantially less than the vertical height or rise of thearch unit and also substantially less than the span of the arch unitbetween the bottoms of the side wall portions. The precast concrete archunit of the invention is reinforced with a predetermined volume ofreinforcing fibers such as steel fibers which provide the arch unit withsubstantial ductility and flexure strength in order to utilize theresistance forces produced by the compacted soil mass surrounding thearch unit. Utilizing the compacted soil to resist bulging of the sidewall portions of the arch unit enables a substantial load to besupported by the top wall portion.

The interaction between the arch unit of the invention with thesurrounding soil in combination with the fiber reinforcement not onlyprovides for eliminating steel reinforcement bars and rods, but alsoprovides for substantially reducing the wall thickness of the precastconcrete arch unit. The reduction in the wall thickness of the arch unitfurther increases the ductility of the unit and facilitates the transferof loads and stresses within the arch unit. Additional economies areachieved by varying the wall thickness of the side wall portions toproportion the strength provided by the arch unit to the applied loadsand stresses for the varying conditions around the perimeter of the archunit. Tapering of the side wall portions also reduces the volume ofconcrete required to cast the arch unit, thereby further reducing theweight and production cost of the arch unit. In a modification of theinvention, one or more elongated bands of longitudinally extendingreinforcing fibers are connected or bonded to peripheral or arcuatesections of the top and side wall portions.

Other features and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plurality of underground open bottomprecast arch units constructed in accordance with the invention andshown supported by parallel spaced concrete footers;

FIG. 2 is an end view of the arch unit of FIG. 1 and shown buried incompacted soil;

FIG. 3 is an end view of only the arch unit shown in FIG. 2 andillustrating the radii of curvature for the side and top wall portionsof the arch unit along with the span, rise and wall thicknesses of theunit;

FIG. 4 is a diagrammatic view of the arch unit shown in FIG. 2 andillustrating, in exaggerated form, the deflection of the side and topwall portions in response to an applied downward load; and

FIG. 5 is a perspective view of a precast concrete arch unit constructedin accordance with the invention and having elongated bands oflongitudinally extending reinforcing fibers bonded to the outer surfaceof the arch unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a plurality or series of longitudinally alignedone-piece open bottom fiber reinforced precast concrete arch units 10constructed in accordance with the invention and supported by parallelspaced elongated concrete footers 12 which may be cast in place at theconstruction site or be part of a poured concrete floor. The arch units10 are buried in compacted soil 15 (FIG. 2) and define a passage 16through which pedestrians, vehicles or water may pass. The arch unitsmay also be provided with end walls to define a chamber in which watermay be temporarily stored, for example, run-off water received from alarge parking lot.

Referring to FIGS. 1 & 3, each of the precast concrete arch units 10 hasa length L, for example, eight feet, and includes a pair of curved orarcuate opposite legs or side wall portions 18 integrally connected by acurved or arcuate top wall portion 22, and the arcuate length of eachportion 18 and 22 may be generally the same, as shown by the lines 23and 24. The arch unit 10 has a height or rise R from the bottom surfacesof the side wall portions 18 to the top inner surface of the top wallportion 22, and the arch unit has a span S between the bottom innersurfaces of the side wall portions 18.

Each of the side wall portions 18 has an inner radius of curvature R1,and the top wall portion 22 has an inner radius of curvature R2.Preferably, the radius R1 is greater than twice the radius R2, and therise R is less than the radius R1. The radius R2 is also greater than 20percent of the span S and less than 45 percent of the span. Thus theradius R2 is substantially smaller than the rise R, and the radius R1may be greater than the span S, as shown in FIG. 3. When desired, thearcuate length of each side wall portion 18 may be shortened to reducethe rise R by relocating or inserting end walls within the arcuate formsused to precast the arch unit 10.

Each of the precast concrete arch units 10 is poured with apredetermined volume of reinforcing fibers F (FIG. 1) which arepreferably steel with each fiber having a length preferably within arange of 0.25 inch to 3 inches and a length at least fifty times thecross-sectional thickness of the fiber. The density of the fibers asthey are thoroughly mixed into the concrete before pouring the precastconcrete arch unit 10 is preferably within a range of 0.25 and 2.0percent by volume. One source of steel fibers which has providedsatisfactory results are produced by Polytorx, LLC in Ann Arbor, Mich.and sold under the trademark HELIX. The twisted steel fibers of thisCompany are disclosed in U.S. Pat. No. 6,060,163. However, other fibersalso provide satisfactory results, such as the steel fibers produced byBekaert Corporation and sold under the trademark DRAMIX. Synthetic ornon-metal fibers may also be used, for example, fibers sold by Nycon,Inc. under the trademark NYCON.

Each of the arch units 10 does not have any embedded steel reinforcingbars or rods, but has a relatively thin wall thickness to provide thefiber reinforced side wall portions 18 and top wall portion 22 withductility and flexure strength. As shown in FIG. 3, the side wallportions 18 may also be tapered along its arcuate length from the topwall portion 22 which has a uniform thickness TT. The thickness TT ofthe top wall portion 22 may thus be greater than the thickness TB at thebottom or base of the side wall portions 18. Preferably, the thicknessTB is within the range of 1.5 percent to 3.5 percent of the span S andwithin a range of 0.5 to 0.9 of the top wall thickness TT. For example,in an arch unit 10 having a span S of 16 feet, the top wall thickness TTmay be within the range of 5 inches to 7.5 inches while the bottom wallthickness TB may be in the range of 2.5 inches to 4 inches.

Referring to FIG. 4, an open bottom precast concrete arch unit 10 buriedwithin the soil 15 is diagrammatically illustrated by a single line.When a downward load W is applied to the soil above the arch unit 10,for example, by the wheels of a heavy vehicle, the top wall portion 22tends to deflect downwardly, causing the side wall portions 18 todeflect or bulge outwardly, as greatly exaggerated in FIG. 4. However,the resistance forces produced by the compacted soil 15, as indicated bythe arrows 30 substantially eliminates or minimizes the bulging of theside wall portions 18 with the result that the deflection of the topwall portion 22 is substantially eliminated or minimized. As a result,the possibility of cracking the fiber reinforced precast concrete archunit 10 in response to a load W is substantially reduced.

Referring to FIG. 5, the wall thickness of the arch unit 10 or of anyreinforced concrete arch unit may be minimized and/or the arch unit maybe designed to support a heavier load W by adhesively bonding to theouter peripheral or arcuate surface of the arch unit one or moreelongated bands 35 of longitudinally extending continuous fibers such asnon-metal fibers or carbon fibers. A suitable epoxy adhesive may be usedto form a positive bond between the bands 35 and the outer surface ofthe arch unit 10, and the bands need to extend peripherally over atleast part of each of the side wall portions and part of the top wallportion in order to overlap the areas of the arch unit which receive thehighest bending stress. However, each band may be more conveniently andquickly attached by having one continuous band 35 which also extendsover the top wall portion, as shown in FIG. 5, instead of attachingshorter sections of the bands. Preferably, two peripherally extendingthin bands 35 are attached adjacent opposite ends of the arch unit, asshown in FIG. 5, but may be spaced somewhat inwardly from the ends ofthe unit to provide for the attachment of strips of joint cover materialwhen two arch units abut each other, as shown in FIG. 1. Each of thebands 35 may also have a second overlapping band 38 which may be shorterand applied only in each area of maximum bending stress.

As apparent from the drawings and the above description, an open bottomfiber reinforced precast concrete arch unit constructed in accordancewith the invention provides desirable features and advantages. Forexample, by eliminating conventional reinforcing bars and rods and usingreinforcing fibers within an arch unit and substantially reducing thewall thickness of the arch unit, the arch unit is provided withductility and flexure strength so that it may utilize the resistanceforces exerted by the compacted surrounding soil to enable the arch unitto support a substantial load applied to the soil above the unit. Inaddition, the replacement of steel reinforcing bars or rods with thereinforcing fibers F and the substantial reduction in the wall thicknessof the arch unit results in a substantial reduction in the volume ofconcrete used to form the arch unit and a corresponding substantialreduction in the weight of the arch unit. These reductions provide for asignificant reduction in the cost of producing the precast arch unit aswell as the cost of handling, transporting and installing the arch unitat the construction site. Moreover, by avoiding the use of steelreinforcing bars or rods, the labor required for positioning thereinforcing bars or rods between the form surfaces for the arch unit, iseliminated.

Additional economies are achieved by varying the thickness of the sidewall portions 18 of the arch unit 10 to proportion the strength providedby the arch unit according to the applied load and stresses around theperimeter of the arch unit. Bending stresses caused by concentratedloads to the top of the arch unit are transformed into thrusts that canbe accommodated very efficiently in the arch unit. This transfer ofloads and stresses is dramatically enhanced by increasing the ductilityof the arch structure. Since the bending stresses in the arch unitdiminish towards the base or bottom of the arch unit, the thickness ofthe side wall portions may be reduced towards the bottoms of the sidewall portions. As another advantage, the discontinuity of the steelfibers F prevents corrosion of the reinforcing fibers, and thiseliminates the need for additional concrete to protect the reinforcementas is required to protect continuous steel reinforcing bars and rods.Also the fibers add toughness to the precast arch unit, and thetoughness resists damage to the unit during handling, shipping andinstalling. It is also apparent that a plurality of the arch units 10shown in FIGS. 2 and 3 may be nested during storage and shipping toprovide additional economies. It is also apparent that a plurality oflaterally adjacent rows of the arch units as shown in FIG. 1 may be usedfor an underground water storage facility.

While the form of arch unit and its method of construction and useherein described constitute a preferred embodiment of the invention, itis to be understood that the invention is not limited to this preciseform of arch unit and method, and that changes may be made thereinwithout departing from the scope and spirit of the invention as definedin the appended claims.

1. An underground open bottom precast concrete arch unit adapted to beburied in compacted soil, said arch unit comprising a one-piece invertedU-shaped wall structure having a predetermined length and arcuateopposite side wall portions integrally connected by an arcuate top wallportion, a predetermined volume of reinforcing fibers dispersedthroughout said wall structure and providing said side wall portions andsaid top wall portion with a predetermined ductility and flexurestrength adapted to utilize the resistance of the surrounding compactedsoil to increase the load capacity of the unit, said wall structurehaving a rise defined between bottom surfaces of said side wall portionsand an inner top surface of said top wall portion, said arcuate top wallportion having a radius of curvature substantially smaller than saidrise, and said arcuate side wall portions each having a radius ofcurvature greater than said rise.
 2. A precast concrete arch unit asdefined in claim 1 wherein said arcuate top wall portion has a radius ofcurvature less than one-half of said radius of curvature of said arcuateside wall portions.
 3. A precast concrete arch unit as defined in claim2 wherein said top wall portion has an arcuate length generally the sameas the arcuate length of each of said side wall portions.
 4. A precastconcrete arch unit as defined in claim 1 wherein each of said arcuateside wall portions has inner and outer curved surfaces convergingtogether toward said bottom surface of said side wall portion andproviding said side wall portion with a tapered wall configuration.
 5. Aprecast concrete arch unit as defined in claim 4 wherein each of saidside wall portions has a bottom thickness less than ninety percent ofthe thickness of said top wall portion.
 6. A precast concrete arch unitas defined in claim 1 wherein said volume of reinforcing fibers withinsaid wall structure is within a range of 0.25 and 2.0 percent by volumeof said wall structure.
 7. A precast concrete arch unit as defined inclaim 1 wherein said arcuate top wall portion has a radius of curvaturegreater than twenty percent of the span of said unit as defined betweeninner bottom surfaces of said side wall portions and less than fortypercent of said span.
 8. A precast concrete arch unit as defined inclaim 1 wherein said reinforcing fibers within said wall structurecomprise steel fibers having a length within a range of 0.25 inch and 3inches.
 9. A precast concrete arch unit as defined in claim 1 whereineach of said reinforcing fibers within said wall structure has a lengthat least fifty times the cross-sectional thickness of said fiber.
 10. Aprecast concrete arch unit as defined in claim 1 wherein each of saidwall portions of said wall structure has a bottom thickness within arange of 1.5 percent to 3.5 percent of the span of said wall structureas defined between bottom inner surfaces of said side wall portions. 11.A precast concrete arch unit as defined in claim 1 and including atleast one elongated band of longitudinally extending reinforcing fibersconnected to peripheral portions of said arcuate top and side wallportions.
 12. A precast concrete arch unit as defined in claim 11wherein said band of reinforcing fibers extends across an area wheresaid top wall portion integrally connects with at least one of said sidewall portions.
 13. A precast concrete arch unit as defined in claim 11wherein a plurality of parallel spaced said bands are adhesively bondedto an outer surface of said arcuate top and side wall portions adjacentopposite ends of said wall structure.
 14. A precast concrete arch unitas defined in claim 11 and including a second said band overlying andadhesively bonded to at least a portion of the first said band.
 15. Anunderground open bottom precast concrete arch unit adapted to be buriedin compacted soil, said arch unit comprising a one-piece invertedU-shaped wall structure having a predetermined length and arcuateopposite side wall portions integrally connected by an arcuate top wallportion, a predetermined volume of reinforcing fibers dispersedthroughout said wall structure and providing said side wall portions andsaid top wall portion with a predetermined ductility and flexurestrength adapted to utilize the resistance of the surrounding compactedsoil to increase the load capacity of the unit, said wall structurehaving a rise defined between bottom surfaces of said side wall portionsand an inner top surface of said top wall portion, said arcuate top wallportion having a radius of curvature substantially smaller than saidrise and between twenty percent and forty five percent of the span ofsaid unit as defined between inner bottom surfaces of said side wallportions, and said arcuate side wall portions each having a radius ofcurvature greater than said rise.
 16. A precast concrete arch unit asdefined in claim 15 wherein said arcuate top wall portion has a radiusof curvature less than one-half of said radius of curvature of saidarcuate side wall portions.
 17. A precast concrete arch unit as definedin claim 15 wherein each of said arcuate side wall portions has innerand outer curved surfaces converging together toward said bottom surfaceof said side wall portion and providing said side wall portion with atapered wall configuration.
 18. A precast concrete arch unit as definedin claim 15 wherein said volume of reinforcing fibers within said wallstructure is within a range of 0.25 and 2.0 percent by volume of saidwall structure.
 19. An underground open bottom precast concrete archunit adapted to be buried in compacted soil, said arch unit comprising aone-piece inverted U-shaped wall structure having a predetermined lengthand arcuate opposite side wall portions integrally connected by anarcuate top wall portion, said side wall portions and said top wallportion having ductility and flexure strength adapted to utilize theresistance of the surrounding compacted soil to increase the loadcapacity of the unit, said wall structure having a rise defined betweenbottom surfaces of said side wall portions and an inner top surface ofsaid top wall portion, and at least one elongated band of longitudinallyextending reinforcing fibers connected to an outer surface of saidarcuate top and side wall portions and extending across an area wheresaid top wall portion integrally connects with at least one of said sidewall portions.
 20. A precast concrete arch unit as defined in claim 19wherein a plurality of parallel spaced said bands are adhesively bondedto an outer surface of said arcuate top and side wall portions adjacentopposite ends of said wall structure.
 21. A precast concrete arch unitas defined in claim 19 and including a second said band overlying andadhesively bonded to at least a portion of the first said band.
 22. Anunderground open bottom precast concrete arch unit adapted to be buriedin compacted soil, said arch unit comprising a one-piece invertedU-shaped wall structure having a predetermined length and arcuateopposite side wall portions integrally connected by an arcuate top wallportion, a predetermined volume of reinforcing fibers dispersedthroughout said wall structure, said volume of reinforcing fibers withina range of 0.25 and 2.0 percent by volume of said wall structure withsaid fibers having a length within a range of 0.25 inch and 3 inches toprovide said side wall portions and said top wall portion with apredetermined ductility and flexure strength adapted to utilize theresistance of the surrounding compacted soil to increase the loadcapacity of the unit, said wall structure having a rise defined betweenbottom surfaces of said side wall portions and an inner top surface ofsaid top wall portion, and said arcuate top wall portion having a radiusof curvature substantially smaller than a radius of curvature of each ofsaid side wall portions.